Convert Bogen to guitar amp?

Flatpicker

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Jun 3, 2004
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I?ve had this Bogen CHA-10 lying around for about 10 years and finally decided to take a look at it yesterday. I?d like to turn it into a nice little low wattage guitar amp without having to completely rebuild it, if possible.

If it used 12AX7(s) like most other Bogen amps, it would be simple, but this thing uses a 6AV6 triode for the mic input and a 6AU6 pentode for the gain stage. It also uses feedback and a 5-position switch to form an unique tone control that I have never seen before. Other than that, the rest looks like fairly common stuff.

I noticed that the 6BW4?s heater voltage uses the same winding as the other tubes. Does this make it prone to more hum or something?

Here?s a link to the schematic
 

NewYorkDave

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As long as the amp's working properly, you can use it for guitar as-is. Just plug into the mic input jack and go! You might like it just as it is. If not, then come back here and we'll talk about mods.

Oh, do add a grounded power cord and a fuse, for safety. The schematic shows no fuse, which is a bad idea indeed
 

Flatpicker

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Thanks for the info, guys!

The mic jack is the old style screw-on type, so I?ll have to remove it and install a 1/4" RCA. Also, the mic jack is isolated from the chassis via a small round piece of phenolic material. I was reading somewhere that the guitar jack should be isolated here as well, which would be easy to do since the chassis hole is rather large. Since I?m replacing the jack, should I go ahead and replace the .033uF/3.3M with the generic 1M/68K used on guitar tube amps?

BTW, just curious, but how can the grid be biased negative with the cathode grounded? Shouldn?t there be some R with C bypass from cathode to ground?

Other than adding a jack, I will replace the line cord with a grounded one and add a 1A slo-blo fuse before powering it up. This thing is old, so I'm sure I'll need to replace the ele's.

I can throw up some pictures if anyone is interested.
 

Flatpicker

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[quote author="NewYorkDave"]...Read up on "grid leak bias" if you want to know why.[/quote]Will do. Thanks.
 

ToobieSnack

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Mar 3, 2005
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493
i got this nice lil bogen mx60a...
nice lil amp/mixer..
it has a limiter built in .... and a place inside to connect a remote for limiter settings ....
i hveben thenking of doing the same thing ..
nice looking iron in these baby's ..
hoopw it works out for you.
later ts
 

Flatpicker

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Jun 3, 2004
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Location
Tennessee
Yep, the Bogen's have lot's of nice iron and they are cheap!! I decided that I should probably make me a speaker cab before I went any futher, so it's on hold for the moment. I'll let you know how it turns out.
 

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